Recently, based on the development of optical technologies, display techniques using various schemes, such as a plasma display panel (PDP), a liquid crystal display (LCD), and the like, replacing the conventional Cathode Ray Tube, have been proposed and have become available on the market.
Advanced level requirements are required for a polymer material used for such displays. For example, in case of LCDs, as LCDs are becoming thinner, lighter, and larger in terms of screen area, obtaining a wide viewing angle and a high contrast ratio, suppressing of a change in an image color according to a viewing angle, and making a screen display uniform have become particularly significant issues.
Accordingly, various polymer films are used for a polarization film, a polarizer protective film, a retardation film, a plastic substrate, a light guide plate, and the like, and various modes of LCD devices using twisted nematic (TN), super-twisted nematic (STN), vertical alignment (VA), in-plane switching (IPS) liquid crystal cells, and the like, as liquid crystal have been developed. These liquid crystal cells have a unique liquid crystal arrangement, respectively having a unique optical anisotropy, and, in order to compensate for the optical anisotropy, films obtained by stretching various types of polymers and providing retardation function thereto have been proposed.
A polarization plate generally has a structure in which a triacetyl cellulose film (TAC) film is laminated on a polarizer as a protective film, using a water-based adhesive made of a polyvinylalcohol-based aqueous solution. In this respect, however, neither the polyvinylalcohol film used as a polarizer nor the TAC film used as a polarizer protective film have sufficient heat and moisture resistance. Thus, when the polarization plate comprised of the films is used for a long period of time in high temperature or high moisture conditions, the degree of polarization may be degraded, the polarizer and the protective film may be separated, and optical characteristics may be degraded, such that the polarization plate is variably restricted in terms of the purpose thereof.
Also, the TAC film has an in-plane retardation value (Rin) and a thickness retardation value (Rth) which are severely changed according to changes in ambient temperature and moisture in the surrounding environment, and, in particular, a retardation value with respect to incident light in a tilt direction is greatly varied. Thus, an application of the polarization plate including the TAC film having the foregoing characteristics as a protective film to an LCD device would result in a degradation of image quality due to the viewing angle characteristics thereof being changed according to a change in the ambient temperature and moisture environment.
Also, the TAC film has a great dimensional variation according to changes in ambient temperature and moisture environments and a relatively high photoelastic coefficient value. Thus, after an evaluation of durability in a heat-resistant, moisture resistant environment, retardation characteristics may be changed locally to thereby degrade image quality. An acrylic resin has been well known as a material for complementing the various shortcomings of the TAC film. However, the acrylic resin does not have sufficient heat resistance characteristics, and creates a retardation in in-plane and thickness directions after being stretched, and therefore is not appropriate to be applied as a protective film.
Thus, in order to solve the problem of the related art, a method for preparing an acrylic copolymer resin having little retardation in the in-plane and thickness directions, a small photoelastic coefficient, and excellent heat resistance, even after being stretched, to be used as a polarizer protective film, is required.